Fluctuation noise in vacuum tubes which are not temperature-limited

This paper is concerned chiefly with a discussion of whether or not the anode stream of a thermionic valve produces a component of fluctuation voltage equal to that which would be produced by a metallic conductor of equal resistance at the cathode temperature. In a recent paper, G. L. Pearson has shown that when the anode current is very small, the fluctuation voltage is equal to that in a metallic conductor of equal resistance at a temperature about half that of the cathode. In this paper it is shown that when the anode current is very small a familiar expression for the shot voltage assumes a form suggestive of a thermal-agitation voltage in a resistance at half the cathode temperature. This form is due to the dependence of anode current on cathode temperature when the anode is negative. Thus it would seem that Pearson's work does not necessarily prove that the anode stream is the seat of a thermal-agitation voltage: his results can equally well be interpreted as a shot-voltage effect, and such interpretation gives a direct explanation of why the relevant temperature is only half that of the cathode.